This invention relates generally to spectroscopy and, in particular, to a compact spectrometer capable of a broad spectral range.
As described in the above referenced patent, it is desirable to have a simple, compact spectrometer which has a narrow passband and can be used over a broad spectral range. Prism or grating spectrometers may be used to analyze a wide spectral range but require a large physical size to achieve sufficient separation of spectra. While these types of spectrometers may be made smaller, a loss of spectral resolution will occur. Fabry-Perot spectrometers utilizing etalons are capable of very high resolution with small size, but an individual etalon cannot cover a broad spectral range.
In the above-cited patent, a spectrometer that has small dimensions, a narrow passband, and that can be used over a broad spectral range is disclosed. The desired passband of this spectrometer is adjusted by altering the angle of incident of incoming light to the spectrometer. This can either be performed by rotating the spectrometer while maintaining the direction of incoming light or by maintaining the orientation of the spectrometer and by altering the direction of the incoming light or by a combination of these.
In some applications, it may be considered advantageous to adjust the passband of the above described spectrometer by neither changing the orientation of the spectrometer nor by changing the direction of the measured light. It is therefore considered desirable to have an even greater choice of adjusting the passband of the spectrometer of the above-cited patent.
As in the previously patented invention, collimated, P-polarized light is made incident on an input surface of an optically transparent material at an angle xcex8INC. The light is transmitted through this input transparent material, and reaches a boundary surface between the input material and an output optically transparent material. The material of the input section is preferably highly dispersive, making Snell component values (n sin xcex8) at the boundary surface markedly different for different wavelengths (colors). The material of the output optically transparent material is preferably of low dispersion and high birefringence, such as is characteristic of calcite, for example. The output optically transmissive material is oriented so that its optic axis is other than normal to the boundary surface and is aligned to maximize the birefringence effect of the material.
As in the previous embodiment of the invention described in the above referenced patent, there will be only one wavelength present at the boundary surface that has a Snell component value (n sin xcex8) that is tangent to its corresponding index surface in the output section of the invention. Within this output section, the ray vector, (r), for this wavelength, is parallel to the boundary surface.
Because optical energy propagates in the direction of the ray vector, only the narrow range of wavelengths having ray vectors that are substantially parallel to the boundary surface are able to reach an output surface at an end of the output section. This narrow range of wavelengths comprises the passband which is incident on the detector.
In the above cited patent, tuning of the spectrometer to permit different colors to reach the detector is accomplished by changing the incident angle, xcex8INC, of the light reaching the spectrometer. In the example of the invention described in the above-cited patent, a solid material for the input section is a described. In this solid input material embodiment, tuning is accomplished either by changing the direction of the incident light or by rotating the device. A change in xcex8INC changes the length (n sin xcex8) of all of the colors at the boundary surface of the device. This controls the wavelength whose ray vector is parallel to the boundary in the upper section and, thus, selects which wavelengths are included in the spectrometer""s passband.
In the invention described herein, the spectrometer of the above referenced patent is modified so that the angle of incidence of light upon a fixed spectrometer need not be altered to change the passband of the spectrometer. Movement of the spectrometer with respect to the incident light is also unnecessary to perform passband adjustment. Passband adjustment is instead permitted by using a voltage tunable liquid crystal material. It is possible however to combine reorientation of the incidence light in conjunction with voltage tuning to extend the spectrometer""s passband tuning options.
Other objects, advantages and new features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanied drawings.